Lithium iron phosphate, LiFePO4 (LFP), is the main representative of a family of LiFe(M)XO4 compounds having the olivine structure capable to insert and de-insert Li+ ion when used in a cathode of a lithium battery.
Several processes have been described to make electrochemically active LiFePO4 or partially substituted LiFePO4. In fact, most of such processes allow to make LFP that is more or less substituted by replacing part of the Fe by other metals such as Mn or Mg or by replacing part of the P by another oxyanion forming element such as S, Si, B, Mo or V for example to give: LiFe(M)XO4.
Known processes include solid-state reactions of finely dispersed reactants (WO 02/27823 A1) as wee as solvent assisted precipitation of LiFePO4 (US 2004/0151649 A1). The solid-state process leads to a product that contains secondary phases (such as Fe3P, Fe2P, FeP, LiPO3, Li4P2O7, Fe2P2O7, Li3Fe2(PO4)3, etc.) or dispersed Fe2O3 depending of the reducing conditions, temperature, reactants excess to stoichiometric proportions or reactant dispersion quality. The product obtained by solvent assisted precipitation contains structural defects or other off-composition defects depending on the operating conditions.
Synthesis in the molten state has also been described (WO 2005/062404 A1, further refined in WO 2013/177671 A1). This melt process uses precursors that are more commonly available commercially. This molten process is rapid and not reactant specific since in the molten state and under strongly reducing conditions (in the presence of C, CO or H2). LiFePO4 can be obtained and crystallized upon cooing. This process has been shown to lead to electrochemically active LiFePO4 cathode powder when reduced to powder; but minor components are present, such as Fe3P, Fe2P, FeP, LiPO3, Li4P2O7, Fe2P2O7, Li3Fe2(PO4)3, etc.
Some of these off-composition phases (for example Fe2O3, Fe2P or LiPO3) are undesirable as they can be detrimental to the cycling properties of the overall battery. They cannot be eliminated or controlled with the existing processes.